A dog clutch is engaged to transmit torque by moving two sets of teeth together to intermesh with one another. A dog clutch may be engaged in a random combination of positions as both sets of teeth circumscribe two rotating shafts that are to be brought together for common rotation by engagement of the clutch. If the teeth are not closely set, there will be a significant amount of uncontrolled motion in the clutch when it is engaged, creating noise in a drivetrain utilizing the clutch. Therefore, in order to minimize the amount of uncontrolled motion when the dog clutch is engaged, the two sets of teeth should be closely set relative to one another when the dog clutch is engaged. This requires that the two sides of the dog clutch be very nearly aligned with one another and therefore synchronized, i.e., turning at the same speed, for engagement to be successful.
Manual transmissions typically have a plate clutch that releases the transmission input shaft from the engine when disengaged. A dog clutch with one side (i.e., one of the sets of teeth) connected to the transmission input shaft then has only a few components connected thereto, and therefore a relatively small amount of inertia and resistance to rotational movement. This allows a synchronizer to be employed to synchronize that side of the dog clutch to rotate at the same speed as the other side of the dog clutch, allowing for smooth and reliable engagement of the dog clutch.
The implementation of dog clutches in other types of transmissions, such as electrically-variable transmissions, has thus far been prevented because relatively heavy components, such as a motor/generator with its relatively large inertia, would be connected to either side of the dog clutch. Additionally, in a vehicle with an electrically-variable transmission, the side of the dog clutch operatively connected to the wheels on a vehicle would sometime be subjected to strong random torque inputs when the vehicle is riding over a bumpy surface. Typical synchronizers would not be able to synchronize the speeds of both sides of the dog clutch under such conditions.